1. Field of the Invention
The present invention relates to an organic electroluminescent display device and a method of fabricating an organic electroluminescent display device, and more particularly, to a dual panel-type organic electroluminescent device and a method of fabricating a dual panel-type organic electroluminescent display device.
2. Discussion of the Related Art
Among various different types of flat panel display (FPD) devices, organic electroluminescent display (OELD) devices have been developed because of their light-emitting properties, wide viewing angles, and good contrast ratios, as compared with the liquid crystal display (LCD) devices. Since a backlight device is not necessary in the OELD devices, the OELD devices can be light weight and thin. In addition, the OELD devices have low power consumption. When driving OELD devices, a low voltage direct current (DC) can be used, whereby rapid response speeds can be obtained. Since the OELD devices are solid state devices, unlike the LCD devices, they are sufficiently strong to withstand external impact and have greater operational temperature ranges. In addition, the OELD devices can be manufactured at low costs. For example, since only deposition and encapsulation apparatus are necessary for manufacturing the OELD devices, manufacturing processes of the OELD devices are simple in contrast to the LCD devices and in contrast to plasma display panel (PDP) devices.
During an operation method for the OELD devices, a passive matrix operating method that does not use additional thin film transistors (TFTs) is used. However, since passive matrix-type OELD devices have limited resolution, high power consumption, and reduced operational lifetime, active matrix-type OELD devices have been developed as next generation display devices that require high resolution and large display area.
In passive matrix-type OELD devices, scanning lines and signal lines are arranged to perpendicularly cross each in a matrix configuration, whereas in the active matrix-type OELD devices, a TFT is disposed at each pixel region to function as a switch to turn a first electrode connected to the TFT ON and OFF, and a second electrode is provided to face the first electrode.
In the passive matrix-type OELD devices, a scanning voltage is sequentially supplied to the scanning lines to operate each pixel. Accordingly, in order to obtain a required average brightness, an instantaneous brightness of each pixel during the selection period should reach a value resulting from multiplying the average brightness by the total number of scanning lines. Thus, since supplied voltage and current increase as the total number of the scanning lines increases, the passive matrix-type OELD devices are not adequate to display high resolution images over a large display area due to the high power consumption.
However, in active matrix-type OELD devices, a voltage supplied to the pixel is stored in a storage capacitor, thereby maintaining the voltage and driving the device until a voltage of next frame is supplied regardless of the total number of the scanning lines. Accordingly, an equivalent brightness is obtained using low supplied current, wherein the active matrix-type OELD device operates having low power consumption and high image resolution over a large display.
FIG. 1 is an equivalent circuit diagram of a basic pixel structure of an active matrix-type organic electroluminescent display device according to the related art. In FIG. 1, a scanning line 2 is arranged along a first direction, and a signal line 4 and a power line 6 are arranged along a second direction perpendicular to the first direction, thereby defining a sub-pixel region “Psub,” wherein the signal line 4 and the power line 6 are spaced apart form each other. In addition, a switching TFT “TS,” which is commonly referred to as an addressing element, is connected to the scanning line 2 and the signal line 4, and a storage capacitor “CST” is connected to the switching TFT “TS” and the power line 6. A driving TFT “TD,” commonly referred to as a current source element, is connected to the storage capacitor “CST” and the power line 6, and an organic electroluminescent (EL) diode “DEL” is connected to the driving TFT “TD.”
The organic EL diode “DEL” has an organic EL layer between an anode and a cathode. When a forward current is supplied to the organic EL diode “DEL,” an electron and hole are recombined to generate an electron-hole pair through a P-N (positive-negative) junction between the anode, which provides the hole, and the cathode, which provides the electron. Since the electron-hole pair has an energy lower than an energy of the separated electron and hole, an energy difference is created between the recombination and the separated electron-hole pair, whereby light is emitted due to the energy difference.
Two different types of organic EL devices exist according to a direction of light emitted from the organic EL diode: passive matrix-type and active matrix-type.
FIG. 2 is a schematic cross sectional view of a bottom emission-type organic electroluminescent display device according to the related art, wherein one pixel region includes red, green, and blue sub-pixel regions. In FIG. 2, first and second substrates 10 and 30 face and are spaced apart from each other, wherein a peripheral portion of the first and second substrates 10 and 30 are sealed together by a seal pattern 40. A thin film transistor (TFT) “T” is formed at each sub-pixel region “Psub” on an inner surface of the first substrate 10, and a first electrode 12 is connected to the TFT “T.” In addition, an organic electroluminescent layer 14 including luminescent materials of red, green, and blue is formed on the TFT “T” and the first electrode 12, and a second electrode 16 is formed on the organic electroluminescent layer 14. Accordingly, the first and second electrodes 12 and 16 supply an electric field to the organic electroluminescent layer 14. Although not shown, an adhesive and a moisture absorbent are formed on an inner surface of the second substrate 30 to shield the substrates from external moisture.
In the bottom emission-type OELD device, for example, the first electrode 12 functions as an anode and is made of a transparent conductive material, and the second electrode 16 functions as a cathode and is made of a metallic material of low work function. In addition, the organic electroluminescent layer 14 is composed of a hole injection layer 14a, a hole transporting layer 14b, an emission layer 14c, and a electron transporting layer 14d over the first electrode 12. The emission layer 14c has a structure where emissive materials of red, green, and blue are alternately disposed at each sub-pixel region “Psub.”
FIG. 3 is a schematic cross sectional view of a sub-pixel region of a bottom emission-type organic electroluminescent display device according to the related art. In FIG. 3, a thin film transistor (TFT) “T” having a semiconductor layer 62, a gate electrode 68, and source and drain electrodes 80 and 82 is formed on a substrate 10. The source and drain electrodes 80 and 82 are connected to a power electrode 72 that extends from a power line (not shown) and an organic electroluminescent (EL) diode “DEL,” respectively. In addition, a storage capacitor “CST” includes the power electrode 72 and a capacitor electrode 64 facing each other with an insulating layer disposed between the power electrode 72 and the capacitor electrode 64, wherein the capacitor electrode 64 is made of the same material as the semiconductor layer 62.
In FIG. 3, the TFT “T” and the storage capacitor “CST” are collectively referred to as array elements “A,” whereas the organic EL diode “DEL” includes first and second electrodes 12 and 16 that face each other with an organic EL layer 14 disposed therebetween. The source electrode 80 of the TFT “T” is connected to the power electrode 72 of the storage capacitor “CST,” and the drain electrode 82 of the TFT “T” is connected to the first electrode 12 of the organic EL diode “DEL.”
FIG. 4 is a flow chart showing a fabricating process of an organic electroluminescent display device according to the related art. In FIG. 4, at step st1, array elements are formed on a first substrate, wherein the array elements include a scanning line, a signal line, a power line, a switching TFT, and a driving TFT. In addition, the signal line and the power line cross the scanning line and are spaced apart from each other, wherein the switching TFT is disposed at the crossing of the scanning and signal lines and the driving TFT is disposed at the crossing of the scanning and the power lines.
At step st2, a first electrode of an organic EL diode is formed over the array elements, wherein the first electrode is connected to the driving TFT of each sub-pixel region.
At step st3, an emission layer of the organic EL diode is formed on the first electrode. If the first electrode is designed to function as an anode, the organic EL layer can be composed of a hole injection layer, a hole transporting layer, an emission layer, and an electron transporting layer.
At step st4, a second electrode of the EL diode is formed on the organic EL layer. The second electrode is formed over an entire surface of the first substrate to function as a common electrode.
At step st5, the first substrate is encapsulated with a second substrate, wherein the second substrate protects the first substrate from external impact and prevents damage of the organic EL layer due to ambient air. In addition, a moisture absorbent may be included in an inner surface of the second substrate.
Accordingly, an organic EL device is fabricated through encapsulating the first substrate including the array elements and the organic EL diode with the second substrate. Since the production yield of the array elements multiplied by the production yield of the organic EL diode determines the production yield of the organic EL device, the production yield of the whole process is greatly restricted by the process for forming the organic EL diode. For example, even when the array elements are properly fabricated, the organic EL diode may not be properly fabricated, and thus, the production yield is adversely effected.
The bottom emission-type organic EL device has high encapsulation stability and high process flexibility. However, since the aperture ratio is restricted, it is difficult to incorporate the bottom emission-type organic EL device into a device having high image resolution. On the other hand, since the top emission-type organic EL device is easily designed and has high aperture ratio, the top emission-type organic EL device has some advantages, such as long operational lifetime. However, since the cathode is generally formed on the organic EL layer in the organic EL device of the top emission-type EL device, transmittance is reduced due to limitation of material selection such that optical efficiency is reduced. In addition, when a thin film protection layer is used to minimize the reduction of the transmittance, infiltration of ambient air is increased.